Gpa-grade bainite steel having ultra-high yield ratio and manufacturing method for gpa-grade bainite steel

ABSTRACT

GPa-grade bainite steel having an ultra-high yield ratio, containing, in addition to Fe, the following chemical elements in mass percentages: 0.12-0.24% of C; 0.2-0.5% of Si; 1.3-2.0% of Mn; 0.001-0.004% of B; 0.01-0.05% of Al; and at least one of Cr, Nb, Ti, and Mo, wherein Cr≤0.4%, Nb≤0.06%, Ti≤0.1%, and Mo≤0.4%. Also disclosed are a manufacturing method and annealing process for the steel.

TECHNICAL FIELD

The present disclosure relates to a steel and a method for manufacturingthe same, in particular to a GPa-grade bainite steel and a method formanufacturing the same.

BACKGROUND ART

Under the new-era concept of “green-safety”, GPa-grade high-strengthsteel has become one of the automotive structural materials thatattracts the most interest of the major automobile manufacturers ashigher and higher strength is required for structural parts and safetyparts of automobiles.

In recent years, more and more automotive structural parts (e.g.components of the cockpit and chassis system) are required to ensure“zero deformation” during service to guarantee proper use of theautomobile and safety of the driver and passengers. This imposes veryhigh requirements on the material. Particularly, it is required to haveyield strength or yield ratio as high as possible. At present,high-yield-strength or high-yield-ratio materials have attracted moreand more attention from automobile manufacturers. The market demand forhigh-yield-strength or high-yield-ratio GPa-grade steel has alsoincreased day by day.

The yield ratio of the GPa-grade high-strength steel mentioned in theexisting patents for invention is generally not high. The yield ratio ofthe dual-phase steel accounting for 90% of the market share of theGPa-grade automotive high-strength steel is only 0.6-0.75. For the restproducts accounting for a small proportion, such as martensite steel,quenching-partitioning steel (Q&P steel), and complex phase steel,although the yield ratio has been increased slightly, it is only about0.75 0.85.

For example, the Chinese patent document CN103361577A (published on Oct.23, 2013 and titled “HIGH-YIELD-RATIO AND HIGH-STRENGTH STEEL SHEETHAVING EXCELLENT PROCESSABILITY”) discloses a high-yield-ratio andhigh-strength steel sheet having a microstructure dominated by ferrite,martensite, tempered martensite and bainite. Its tensile strength canreach 980 MPa or higher, but its yield ratio is only ≥0.68, which stillfails to meet the latest requirement of the automotive parts market forhigh-yield-ratio GPa-grade steel sheets.

For another example, the Chinese patent document CN106170574A (publishedon Nov. 30, 2016 and titled “HIGH-YIELD-RATIO AND HIGH-STRENGTHCOLD-ROLLED STEEL SHEET AND MANUFACTURING METHOD THEREFOR”) discloses ahigh-yield-ratio and high-strength cold-rolled steel sheet and amanufacturing method therefor. The structure of the steel sheet mainlycontains ferrite, retained austenite, martensite and trace amounts ofbainite and tempered ferrite. Its tensile strength can reach 980 MPa orhigher, but its yield ratio is only ≥0.75, no more than 0.8 at most. Themarket demand for GPa-grade high-strength steel having a yield ratio≥0.9still cannot be satisfied.

On the other hand, although some patent documents disclosehigh-yield-ratio steel sheets having a yield ratio≥0.9 and manufacturingmethods therefor, the tensile strength of the steel sheets disclosed bythese patent documents cannot reach the level of 980 MPa.

For example, the Chinese patent document CN102719736A (published on Oct.10, 2012 and titled “ULTRAFINE GRAIN SLIDEWAY STEEL HAVING YIELDRATIO≥0.9 AND PRODUCTION METHOD THEREFOR”) discloses a steel sheethaving a yield ratio≥0.9 obtained by forming an ultrafine grainstructure, but its tensile strength is only at the level of 700 MPa.

As it can be seen, for a steel sheet at the present stage, the tensilestrength reaching the GPa grade and the yield ratio≥0.9 are twocontradictory technical indicators. The technical problem behind thiscontradiction is that it is technically very difficult to achieve anultra-high yield ratio of ≥0.9 by regulating the structure.

First of all, to achieve a high yield ratio, the structure of the matrixin the steel sheet needs to be relatively uniform. For example, thematrix consists of pure bainite or pure martensite. If the steel sheethas a multi-phase or complex-phase matrix structure, such as a matrixstructure containing ferrite, retained austenite, tempered martensiteand martensite at the same time, it is not easy to obtain a high yieldratio. However, if it's desired that the strength of the steel sheetreaches the GPa level, the mutual cooperation of multiple phases in thestructure is often required, such as in the typical ferrite/martensitedual-phase steel and the advanced high-strength steel that containsretained austenite and incorporates the TRIP effect. This is the firstaspect of the technical contradiction.

However, even with a pure bainite or martensite structure, due todislocation slip and work hardening caused by processing strain, it'sdifficult for the yield ratio of the steel sheet to reach ≥0.9.Generally speaking, the yield ratio of a steel sheet having a puremartensite or bainite matrix is about 0.8-0.9.

Therefore, in order to further obtain a steel sheet having an ultra-highyield ratio, it is necessary to further design a complex composition anda complex process to prevent dislocation slip and increase the yieldstrength of the material. For example, the Chinese patent documentCN101910436A (published on Dec. 8, 2010 and titled “HIGH-STRENGTHCOLD-ROLLED STEEL SHEET HAVING EXCELLENT WEATHER RESISTANCE ANDPREPARATION METHOD THEREFOR”) discloses a method for increasing theyield strength of a material by introducing a large amount of expensivesolid solution alloy elements Cr, Zr, Co, W, etc. However, consideringthat the process for preparing the existing GPa-gradeultra-high-strength steel is complicated and the amount of alloyelements added to it is already very high, it is still considerablydoubtful whether the above complicated process technology or theaddition of expensive alloy elements for further increasing the yieldratio is suitable for combining with the existing GPa-gradeultra-high-strength steel whose structure is already extremely complex.This is the second aspect of the technical contradiction.

Therefore, a series of technical difficulties such as the first andsecond technical contradictions mentioned above have to be overcome inorder to obtain GPa-grade ultra-high-strength steel having a yield ratioof ≥0.9. This cannot be realized by the existing patent technology.

As such, in order to solve the above problem, it is desirable to obtaina GPa-grade bainite steel having an ultra-high yield ratio. ThisGPa-grade bainite steel has an ultra-high yield ratio, an ultra highstrength, and excellent hole-expanding and bending performances at thesame time. It can be used to prepare automotive structural parts, andrealize the new design concept of “green-safety” for automobiles.

SUMMARY

One object of the present disclosure is to provide a GPa-grade bainitesteel having an ultra high yield ratio. According to the presentdisclosure, a GPa-grade bainite steel having an ultra-high yield ratiocan be obtained by an appropriate design of the chemical composition.The GPa-grade bainite steel has a tensile strength of ≥980 MPa, a yieldstrength of ≥900 MPa, a yield ratio of ≥0.9, and a hole expansion rateof ≥55%. It has an ultra-high yield ratio, an ultra-high strength, andexcellent hole-expanding and bending performances at the same time. Itcan be used to prepare automotive structural parts, and has goodpopularization prospect and application value.

In order to achieve the above object, the present disclosure proposes aGPa-grade bainite steel having an ultra-high yield ratio, comprising thefollowing chemical elements in mass percentages in addition to Fe andunavoidable impurities:

-   -   C: 0.12-0.24%;    -   Si: 0.2-0.5%;    -   Mn: 1.3-2.0%;    -   B: 0.001-0.004%;    -   Al: 0.01-0.05%;    -   at least one of Cr, Nb, Ti and Mo, wherein Cr≤0.4%, Nb≤0.06%,        Ti≤0.1%, Mo≤0.4%.

Further, in the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure, the mass percentages of thechemical element are:

-   -   C: 0.12-0.24%;    -   Si: 0.2-0.5%;    -   Mn: 1.3-2.0%;    -   B: 0.001-0.004%;    -   Al: 0.01-0.05%;    -   at least one of Cr, Nb, Ti and Mo, wherein Cr≤0.4%, Nb≤0.06%,        Ti≤0.1%, Mo≤0.4%; a balance of Fe and other unavoidable        impurities.

The principles for designing the various chemical elements in thetechnical solution of the present disclosure will be described in detailas follows:

C: In the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure, element C is one of the keyelements that control the phase transformation of the structure in thecarbon steel. Meanwhile, element C has a great influence on the strengthof the steel sheet. Element C can form alloy carbides with other alloyelements, thereby increasing the strength of the steel sheet. If thecontent of element C in the steel is lower than 0.12%, the strength ofthe steel will not meet the target requirement; and if the content ofelement C in the steel is higher than 0.24%, it is easy to formmartensite structure and coarse cementite which will deteriorate theperformances of the steel sheet. As such, in the GPa-grade bainite steelhaving an ultra-high yield ratio according to the present disclosure,the mass percentage of C is controlled at 0.12-0.24%.

Of course, in some preferred embodiments, in order to obtain betterimplementation effects, the mass percentage of element C may becontrolled at 0.15-0.20%.

Si: In the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure, element Si is an essential elementfor deoxygenation in steelmaking. It has a certain solid solutionstrengthening effect, and at the same time, it also has a certaininfluence on the formation of bainite (the more element B in the steel,the easier it is to form carbon-free bainite). It should be noted thatwhen the content of element Si in the steel is lower than 0.2%, it isdifficult to achieve sufficient deoxygenation effect; and when thecontent of element Si in the steel is higher than 0.5%, it is easy toform iron oxide scale or tiger stripe color difference, which is notconducive to the surface quality of steel sheets for automobiles. Assuch, in the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure, the mass percentage of Si iscontrolled at 0.2-0.5%.

Mn: In the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure, element Mn is the main additiveelement, and it is one of the key elements that control the phasetransformation of the structure in the steel. It should be noted thatelement Mn is low in cost. It is not only an element for effectivelyimproving the strength of the steel, but also an important element forsolid solution strengthening. However, it should be noticed that thecontent of element Mn in the steel should not be too high. When thecontent of element Mn in the steel is too high, it will deteriorate thecorrosion resistance and welding performance, aggravate the tendency ofgrain coarsening, and reduce the plasticity and toughness of the steel.As such, in the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure, the mass percentage of Mn iscontrolled at 1.3-2.0%.

Of course, in some preferred embodiments, in order to obtain betterimplementation effects, the mass percentage of element Mn may becontrolled at 1.6-2.0%.

B: In the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure, element B is not only beneficial tothe formation of bainite in the steel, but also has a significantinfluence on the strength and hardness of the steel sheet. It should benoted that if the content of element B in the steel is lower than0.001%, the strength of the steel will not meet the target requirement;and if the content of element B in the steel is higher than 0.004%, itis easy to form brittle borides which will affect the hole-expanding andbending performances of the steel sheet. As such, in the GPa-gradebainite steel having an ultra-high yield ratio according to the presentdisclosure, the mass percentage of B is controlled at 0.001-0.004%.

Al: In the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure, element Al is only added to thesteel as a deoxygenating element. It can remove element O from the steelto ensure the performances and quality of the steel. As such, in theGPa-grade bainite steel having an ultra-high yield ratio according tothe present disclosure, the mass percentage of Al is controlled at0.01-0.05%. In some existing technologies, element Al is added to steelin a large amount (≥0.1%) as an element for forming ferrite andinhibiting carbide precipitation in an attempt to effectuate solidsolution strengthening, or to change the phase transformationtemperature, bainite formation kinetics and carbide precipitationkinetics by adding Al, so as to change the phase transformation of thesteel to form retained austenite or carbon-free bainite, therebyimproving the strength of the steel ultimately. However, the compositioncontrol and process adjustment proposed according to the presentdisclosure can already provide a GPa-grade bainite steel having anultra-high yield ratio, so there is no need to add a large amount ofelement Al, thereby avoiding cost increase and greatly increaseddifficulty in steelmaking and manufacturing.

Ti, Cr, Nb and Mo: In the GPa-grade bainite steel having an ultra-highyield ratio according to the present disclosure, Ti, Cr, Nb and Mo areoptional alloy elements that can be added to the steel to form a secondphase of dispersed fine carbide which precipitates to further improvethe strength and yield ratio of the steel sheet. In addition, it shouldbe noted that elements Cr and Mo can prolong the incubation period ofpearlite and ferrite in the CCT curve, inhibit the formation of pearliteand ferrite, and make it easier to obtain the bainite structure duringcooling, which is beneficial to improve the hole expansion rate of thesteel.

As it can be seen, the above four alloy elements have an influence onboth the regulation of the structure of the steel sheet and thecorresponding annealing process. The factors that influence theformation of carbides have a direct influence on the proportion andmorphology of the carbides that are formed. As such, in the GPa-gradebainite steel having an ultra-high yield ratio according to the presentdisclosure, the mass percentage of Cr, Nb, Ti and Mo are respectivelycontrolled as follows: Cr≤0.4%, Nb≤0.06%, Ti≤0.1%, Mo≤0.4%.

On the other hand, the addition of the above alloy elements willincrease the material cost. To balance the performances and the costcontrol, in the technical solution according to the present disclosure,it is preferable to add at least one of Cr, Nb, Ti and Mo to the steel.In some preferred embodiments, the GPa-grade bainite steel having anultra-high yield ratio according to the present disclosure comprises atleast 0.1-0.4% Cr. In some preferred embodiments, the GPa-grade bainitesteel having an ultra-high yield ratio according to the presentdisclosure comprises at least 0.1-0.4% Mo. In some preferredembodiments, the GPa-grade bainite steel having an ultra-high yieldratio according to the present disclosure comprises at least one or bothof Cr and Mo. In some preferred embodiments, the GPa-grade bainite steelhaving an ultra-high yield ratio according to the present disclosurecomprises at least 0.1-0.4% Cr and 0.1-0.4% Mo.

Further, in the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure, the mass percentages of thechemical elements satisfy at least one of the following:

-   -   C: 0.15-0.20%,    -   Mn: 1.6-2.0%.

Further, in the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure, among the other unavoidableimpurities: P<0.015%; and/or S≤0.004%.

In the above technical solution, both P and S are impurity elements inthe steel. If the technical conditions permit, in order to obtain aquenched and tempered steel having better performances and betterquality, the amount of impurity elements in the steel should beminimized.

Further, the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure further comprises at least one ofthe following chemical elements:

-   -   0<Cu≤0.2%, 0<Ni≤0.2%, 0<V≤0.2%, 0<Ce≤0.2%.

In the technical solution according to the present disclosure, each ofthe above Cu, Ni, V and Ce elements can further improve the performancesof the GPa-grade bainite steel having an ultra high yield ratioaccording to the present disclosure.

Further, the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure satisfies 0.18≤M≤0.27, whereinM=Cr/2.5+Ti+V/5+Nb/1.7+Mo/1.7, wherein Cr, V, Nb, Ti and Mo eachrepresent the value in front of the percent sign in the mass percentageof each chemical element.

In the above technical solution, it should be noted that in theGPa-grade bainite steel having an ultra-high yield ratio according tothe present disclosure, while the mass percentages of the chemicalelements are controlled respectively, in order to impart betterperformances and quality to the GPa-grade bainite steel, it's preferredto further control M to be 0.18≤M≤0.27, whereinM=Cr/2.5+Ti+V/5+Nb/1.7+Mo/1.7, wherein Cr, V, Nb, Ti and Mo eachrepresent the value in front of the percent sign in the mass percentageof each chemical element.

It should be noted that in the present disclosure, if M is too high, itwill be easy to form coarse carbides which will deteriorate the holeexpansion rate and bending performance of the steel; and if M is toolow, the carbide precipitate phase cannot be formed sufficiently, sothat the strength and yield ratio of the steel will be insufficient.Therefore, in the present disclosure, M may be controlled to be0.18≤M≤0.27, so as to ensure dispersive precipitation of nano-,submicron- or micron-scale granular carbides in the steel, and ensure amaximum diameter of the granular carbide precipitate.

Further, the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure satisfies 0.20≤Cb≤0.27, wherein theequivalent bainite carbon contentC_(b)=C−(Mo+Nb)/8−(Ti+V)/4−Cr/12+Ni/10+Mn/20+B×10, wherein each elementin the formula represents the value in front of the percent sign in themass percentage of the element.

In the above technical solution, in the GPa-grade bainite steel havingan ultra-high yield ratio according to the present disclosure, since thealloy elements and the M value have an influence on the precipitation ofcarbides, they will indirectly influence the equivalent bainite carboncontent C_(b) in the steel. It should be noted that in the presentdisclosure, if C_(b) is too low, a single bainite matrix structurecannot be formed in a sufficient amount; and if C_(b) is too high, thehardness of bainite will be too large, thereby deteriorating the bendingand hole-expanding performances of the steel. Therefore, in the presentdisclosure, while the mass percentages of the chemical elements arecontrolled respectively, it's preferred to further control C_(b) to be0.20≤C_(b)≤0.27, so as to effectively ensure that the phase proportionof acicular lower bainite in the steel is ≥90%.

Furthermore, in the GPa-grade bainite steel having an ultra-high yieldratio according to the present disclosure, its microstructure is mainlyacicular lower bainite, and the phase proportion of the acicular lowerbainite is ≥90%.

Further, in the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure, its microstructure furthercomprises a nano-, submicron- or micron-scale granular carbideprecipitate phase that is precipitated dispersively, and a total phaseproportion of the granular carbide precipitate phase+acicular lowerbainite is ≥99%.

Furthermore, in the GPa-grade bainite steel having an ultra-high yieldratio according to the present disclosure, the granular carbideprecipitate has a maximum diameter of ≤2 μm.

Further, the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure has a tensile strength of ≥980 MPa,preferably ≥1000 MPa, a yield strength of ≥900 MPa, preferably ≥950 MPa,a yield ratio of ≥0.9, preferably ≥0.95, and a hole expansion rate of≥55%, preferably ≥60%. In a preferred embodiment, the GPa-grade bainitesteel having an ultra-high yield ratio according to the presentdisclosure has a tensile strength of ≥1000 MPa, a yield strength of ≥910MPa, a yield ratio of ≥0.9, and a hole expansion rate of ≥55%.

Further, the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure has a yield strength of ≥950 MPa, ayield ratio of ≥0.95, and further preferably a tensile strength of ≥1000MPa, and a hole expansion rate of ≥60%.

Further, the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure has an elongation of ≥9.0%.Correspondingly, another object of the present disclosure is to providean annealing process for the above GPa-grade bainite steel having anultra-high yield ratio. This annealing process plays a key role for theperformances of the steel. By designing the process appropriately andcontrolling relevant process parameters, the GPa-grade bainite steelhaving an ultra-high yield ratio can be obtained.

In order to achieve the above object, the present disclosure proposes anannealing process for the above GPa-grade bainite steel having anultra-high yield ratio, comprising steps of:

-   -   (a) Heating to a soaking temperature Ts at a heating rate of        ≤50° C./s at a heating stage, wherein Ts is 840-900° C.;    -   (b) Holding the temperature Ts for 5 minutes or less at a        soaking stage;    -   (c) Cooling to (Ts-80) to (Ts-140) ° C. at a first cooling rate        of ≤15° C./s at a slow cooling stage;    -   (d) Cooling to (Ts-490) to (Ts-440) ° C. at a second cooling        rate of ≥(130-Q)° C./s at a fast cooling stage, wherein        Q=C×180+Si×10+Mn×30+Ni×50+Cr×15+Mo×15+B×2000, wherein each        element in the formula represents the value in front of the        percent sign in the mass percentage of the element;    -   (e) Cooling at a third cooling rate for 10-40 s at a controlled        cooling stage for self-temperature rise, wherein        [(Q-80)/12]≤third cooling rate≤[(Q-80)/8];    -   (f) Finally, cooling the strip steel in air to room temperature        at an air cooling stage.

In the technical solution according to the present disclosure, it shouldbe noted that the above annealing process comprises a heating stage, asoaking stage, a slow cooling stage, a fast cooling stage, a controlledcooling stage for self-temperature rise, and an air cooling stage. Itplays a key role for the performances of the GPa-grade bainite steelaccording to the present disclosure.

In step (a), at the heating stage, it is necessary to ensure heating ata heating rate of ≤50° C./s to a soaking temperature Ts: 840-900° C.,preferably heating to a soaking temperature of 840-870° C. The heatingrate at the heating stage should not be too high; otherwise, theuniformity of the structure of the strip steel will be reduced. Inaddition, it should be noted that if the soaking temperature Ts is lowerthan the soaking temperature range designed above, the strip steelcannot acquire ≥90% acicular lower bainite structure; and if the soakingtemperature Ts is higher than the soaking temperature range designedabove, the grains in the strip steel will be coarse, which willdeteriorate the formability of the steel. In some embodiments, theheating rate in step (a) is 5-45° C./s.

In step (b), the soaking time is preferably not less than 1 minute. Forexample, the soaking time is 1 minute to 4.5 minutes.

In step (c), it's necessary to cool the strip steel to (Ts-80) to(Ts-140) ° C. at a first cooling rate of ≤15° C./s at the slow coolingstage. The first cooling rate at the slow cooling stage should not betoo high; otherwise, not only energy waste, but also nonuniformstructure of the strip steel will be resulted. In addition, it should benoted that if the slow cooling temperature is lower than the slowcooling temperature range designed above, the strip steel cannot acquire≥90% bainite structure; and if the slow cooling temperature is higherthan the slow cooling temperature range designed above, a higher coolingcapacity and a higher temperature precision control capability will berequired at the subsequent fast cooling stage, because the structureuniformity of the strip steel will be deteriorated, and in turn, theperformances of the product will be deteriorated if the cooling capacityor temperature precision control capability is insufficient. Preferably,the first cooling rate in step (c) is 5-15° C./s, preferably 5-12° C./s.

In step (d), at the fast cooling stage, it's necessary to cool the stripsteel to (Ts-490) to (Ts-440) ° C. at a second cooling rate of ≥(130-Q)° C./s; wherein Q=C×180+Si×10+Mn×30+Ni×50+Cr×15+Mo×15+B×2000. If thesecond cooling rate at the fast cooling stage is insufficient, or thecooling temperature is higher than (Ts-440) ° C., bainite transformationwill occur prematurely, and a high-temperature bainite structure (suchas upper bainite or equiaxed bainite) will be formed. As a result, notonly a phase proportion of ≥90% of acicular lower bainite in the steelcannot be guaranteed, but the latent heat of phase transformation willalso be reduced greatly, so that subsequent controlled cooling forself-temperature rise cannot be implemented. Thus, the materialstructure will be abnormal, and the steel sheet and steel strip cannotacquire an ultra high yield ratio. Nevertheless, if the coolingtemperature at the fast cooling stage is lower than (Ts-490) ° C., amartensite structure will be formed, and thus the hole expansion rateand bending performance of the steel will be reduced.

In step (e), at the controlled cooling stage for self-temperature rise,if the strip steel can be treated according to the designed parametersat the fast cooling stage, the self-temperature rise phenomenon willoccur to the strip steel due to the release of a large quantity of thelatent heat of phase transformation. The self-temperature rise canincrease the temperature of the strip steel by 50-120° C. rapidly,uniformly and efficiently, thereby promoting uniform and dispersiveprecipitation of carbides. In order to ensure full carbide precipitationand small precipitate size, it is necessary to control the temperatureof the strip steel and cool it at a third cooling rate for 10-40 s,wherein [(Q-80)/12]≤third cooling rate≤[(Q-80)/8].

It should be noted that if the third cooling rate is too low or thecooling time is too long at the controlled cooling stage forself-temperature rise, failing to meet the above design requirementsaccording to the present disclosure, carbide precipitate tends to becoarsened, thereby deteriorating the hole expansion rate and bendingperformance; and if the third cooling rate is too high or the coolingtime is too short, it's likely that carbides cannot precipitate fully,so that the steel cannot acquire an ultra-high yield ratio, i.e. a yieldratio of ≥0.9.

In addition, still another object of the present disclosure is toprovide a manufacturing method for the above GPa-grade bainite steelhaving an ultra-high yield ratio. With the use of this manufacturingmethod, the GPa-grade bainite steel having an ultra-high yield ratioaccording to the present disclosure can be made effectively.

In order to achieve the above object, the present disclosure proposes amanufacturing method for the above GPa-grade bainite steel having anultra-high yield ratio, comprising steps of:

-   -   (1) Smelting and casting;    -   (2) Hot rolling;    -   (3) Post-rolling cooling and coiling;    -   (4) Pickling and cold rolling;    -   (5) The above annealing process.

In the technical solution according to the present disclosure, in theabove manufacturing method, the main purpose of steps (1)-(4) before theannealing process is to obtain a steel sheet or steel strip having auniform composition and a uniform initial structure, so as to ensurethat the uniformity and stability of the structure and performances canbe satisfied in the subsequent annealing process. The annealing processin step (5) plays a key role for the performances of the steel sheet.

Further, in the manufacturing method according to the presentdisclosure, in step (2), a heating temperature is controlled at1150-1260° C.; an initial rolling temperature of finishing rolling iscontrolled at 1100-1220° C.; and a final rolling temperature offinishing rolling is controlled at 900 950° C.

Further, in the manufacturing method according to the presentdisclosure, in step (3), a cooling rate is controlled at 30-150° C./s,and a coiling temperature is controlled at 450-580° C.

Further, in the manufacturing method according to the presentdisclosure, in step (4), a cold rolling reduction rate is controlled at≥50%.

Further, in the manufacturing method according to the presentdisclosure, the GPa-grade bainite steel having an ultra-high yield ratiois the GPa-grade bainite steel having an ultra-high yield ratiodescribed in any embodiment herein.

Compared with the prior art, the GPa-grade bainite steel having anultra-high yield ratio and the method of manufacturing the sameaccording to the present disclosure have the following advantages andbeneficial effects:

On the premise of ensuring that the composition of chemical elements andthe process are relatively simple and controllable, the presentdisclosure optimizes the combination of alloy elements, and adjusts theannealing process in an innovative way. On the basis of ensuring thatthe matrix structure of the steel sheet is a simple and single bainitestructure, the release of the latent heat of phase transformation isintroduced to realize self-temperature rise of the steel strip. This notonly reduces energy consumption, but also realizes fast, uniform andefficient control of the temperature rise of the steel strip, therebyinducing dispersive precipitation of the fine second phase. Thus, aGPa-grade bainite steel having an ultra-high yield ratio and goodformability is obtained.

By designing the chemical composition appropriately according to thepresent disclosure, a GPa-grade bainite steel having an ultra-high yieldratio that has a tensile strength of ≥980 MPa, a yield strength of ≥900MPa, a yield ratio of ≥0.9, and a hole expansion rate of ≥55% can beobtained. The GPa-grade bainite steel has an ultra-high yield ratio, anultra-high strength, and excellent hole-expanding and bendingperformances at the same time. It can be used to prepare automotivestructural parts, and realize the new design concept of “green-safety”for automobiles. It has good popularization prospect and applicationvalue.

The annealing process according to the present disclosure plays a keyrole for the performances of the steel. The annealing process comprisesa heating stage, a soaking stage, a slow cooling stage, a fast coolingstage, a controlled cooling stage for self-temperature rise, and an aircooling stage. By designing the process appropriately and controllingrelevant process parameters, the GPa-grade bainite steel having anultra-high yield ratio can be obtained.

Correspondingly, the manufacturing method according to the presentdisclosure employs a unique production process. Particularly, the aboveannealing process is utilized to guarantee the performances of theresulting GPa-grade bainite steel. The resulting GPa-grade bainite steelnot only has ultra-high strength and yield ratio, but also has excellenthole-expanding and bending performances.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph at 3000× magnification showing the microstructureof the GPa-grade bainite steel of Example 1.

FIG. 2 is a photograph at 3000× magnification showing the microstructureof the comparative steel in Comparative Example 7.

FIG. 3 is a photograph at 1000× magnification showing the microstructureof the comparative steel in Comparative Example 8.

DETAILED DESCRIPTION

The GPa-grade bainite steel having an ultra-high yield ratio accordingto the present disclosure and the manufacturing method for the same willbe further explained and illustrated with reference to the accompanyingdrawings of the specification and the specific Examples. Nonetheless,the explanation and illustration are not intended to unduly limit thetechnical solution of the present disclosure.

Examples 1-14 and Comparative Examples 1-10

The GPa-grade bainite steel having an ultra-high yield ratio in each ofExamples 1-14 was prepared using the following steps:

-   -   (1) Subjecting the chemical composition shown in Table 1 to        smelting and casting.    -   (2) Hot rolling: the heating temperature was controlled at        1150-1260° C.; the initial temperature of the finishing rolling        was controlled at 1100-1220° C.; and the final rolling        temperature of the finishing rolling was controlled at 900-950°        C.    -   (3) Post-rolling cooling and coiling: the cooling rate was        controlled at 30-150° C./s; and the coiling temperature was        controlled at 450-580° C.    -   (4) Pickling and cold rolling: the cold rolling reduction rate        was controlled at ≥50%.    -   (5) Annealing.

It should be noted that in step (5), the annealing process comprises thefollowing steps:

-   -   (a) Heating to a soaking temperature Ts at a heating rate of        ≤50° C./s at a heating stage, wherein Ts was 840-900° C.;    -   (b) Holding the temperature Ts for 5 minutes or less at a        soaking stage;    -   (c) Cooling to (Ts-80) to (Ts-140) ° C. at a first cooling rate        of ≤15° C./s at a slow cooling stage;    -   (d) Cooling to (Ts-490) to (Ts-440) ° C. at a second cooling        rate of ≥(130-Q)° C./s at a fast cooling stage, wherein        Q=C×180+Si×10+Mn×30+Ni×50+Cr×15+Mo×15+B×2000;    -   (e) Cooling at a third cooling rate for 10-40 s at a controlled        cooling stage for self-temperature rise, wherein        [(Q-80)/12]≤third cooling rate≤[(Q-80)/8];    -   (f) Finally, cooling the strip steel in air to room temperature        at an air-cooling stage.

In addition, it should be noted that the GPa-grade bainite steel havingan ultra-high yield ratio in each of Examples 1-14 according to thepresent disclosure was prepared using the above steps. The chemicalcompositions and related process parameters in these Examples all metthe control requirements of the design specification according to thepresent disclosure.

The comparative steel in each of Comparative Examples 1-10 was also madeby the process comprising smelting and casting, hot rolling,post-rolling cooling and coiling, pickling and cold rolling, andannealing. However, the chemical composition and the relevant processparameters in each of Comparative Examples 1-6 included parameters thatfailed to meet the requirements of the design according to the presentdisclosure. Although the chemical composition in each of ComparativeExamples 7-10 met the requirements of the design according to thepresent disclosure, these Comparative Examples all included parametersthat failed to meet the requirements of the design according to thepresent disclosure.

Among the Examples according to the present disclosure and theComparative Examples, Comparative Example 7 and Example 1 had the samecomposition of chemical elements; Comparative Example 8 and Example 2had the same composition of chemical elements; Comparative Example 9 andExample 6 had the same composition of chemical elements; and ComparativeExample 10 and Example 11 had the same composition of chemical elements.

Table 1 lists the mass percentages (%) of the chemical elements in theGPa-grade bainite steel having an ultra-high yield ratio in each ofExamples 1-14 and the mass percentages (%) of the chemical elements inthe comparative steel in each of Comparative Examples 1-10.

TABLE 1 (the balance is Fe and other unavoidable impurities except for Pand S) Steel Chemical elements No. Grade C Si Mn Cr B Mo Nb Ti P S Al CuNi IV Ce C_(b) M Ex. 1 and A 0.16 0.45 1.75 0.3 0.002 0.19 0 0 0.0100.002 0.02 0 0 0 0 0.22 0.23 Comp. Ex. 7 Ex. 2 and B 0.17 0.35 1.82 0.20.004 0.23 0 0 0.012 0.003 0.03 0 0 0 0 0.26 0.22 Comp. Ex. 8 Ex. 3 C0.13 0.5 1.68 0.25 0.004 0.16 0.03 0.03 0.013 0.001 0.01 0 0 0 0 0.200.24 Ex. 4 D 0.19 0.23 2 0.23 0.003 0.13 0.04 0.02 0.011 0.002 0.03 0 00 0 0.27 0.21 Ex. 5 E 0.14 0.27 1.88 0.22 0.004 0.15 0 0 0.010 0.0020.02 0.2 0 0 0 0.24 0.18 Ex. 6 and F 0.17 0.22 1.71 0.3 0.004 0.25 0 00.008 0.001 0.03 0 0 0 0 0.24 0.27 Comp. Ex. 9 Ex. 7 G 0.16 0.43 1.560.21 0.002 0.15 0.02 0.08 0.009 0.001 0.04 0 0 0 0 0.20 10.26 Ex. 8 H0.19 0.41 1.53 0.35 0.002 0.12 0.06 0.01 0.010 0.001 0.05 0 0 0 0 0.230.26 Ex. 9 I 0.18 0.31 1.61 0.38 10.002 0 0 0.1 0.015 0.003 0.04 0 0 0 00.22 0.25 Ex. 10 J 0.2 0.37 1.94 0.25 0.001 0.20 0 0.01 0.010 0.002 0.020 0 0.2 0 0.21 0.27 Ex. 11 K 0.15 0.48 1.58 0 0.003 0.37 0 0 0.008 0.0020.02 0 0 0 0 0.21 0.22 and Comp. Ex. 10 Ex. 12 L 0.22 0.46 1.35 0.160.004 0.31 0.04 0 0.009 0.001 0.03 0 0 0 0.2 0.27 0.27 Ex. 13 M 0.12 0.41.93 0.11 0.003 0.20 0 0.05 0.008 0.004 0.03 0 0.2 0 0 0.22 0.21 Ex. 14N 0.24 0.25 1.3 0.17 0.001 0.27 0.01 0.03 0.010 0.001 0.02 0 0 0 0 0.260.26 Comp. 0 0.25 0.35 2.1 0.35 0.001 0.2 0 0 0.012 0.001 0.02 0 0 0 00.31 0.26 Ex. 1 Comp. P 0.10 0.4 1.7 0.3 0.002 0.25 0 0 0.013 0.003 0.020 0 0 0 0.15 0.27 Ex. 2 Comp. Q 0.18 0.3 1.68 0.28 0.004 0.2 0.02 0.050.011 0.004 0.02 0 0 0 0 0.24 0.29 Ex. 3 Comp. R 0.17 0.22 1.96 0.20.001 0.13 0 0 0.014 0.002 0.02 0 0 0 0 0.25 0.16 Ex. 4 Comp. S 0.150.22 1.51 0.22 0.001 0.3 0 0 0.015 0.002 0.02 0.2 0 0 0 0.18 0.26 Ex. 5Comp. T 0.19 0.22 1.86 0.3 0.004 0.16 0 0.01 0.012 0.003 0.02 0 0 0 00.28 0.22 Ex. 6 Note: in the above table, Cb = C − (Mo + Nb)/−(Ti + V)/4− Cr/12 + Ni/10 + Mn/20 + B × 10, wherein each element in the formularepresents the value in front of the percent sign in the mass percentageof the element; M = Cr/2.5 + Ti + V/5 + Nb/1.7 + Mo/1.7, wherein Cr, V,Nb, Ti and Mo each represent the value in front of the percent sign inthe mass percentage of the chemical element.

Table 2-1 and Table 2-2 list the specific process parameters for theGPa-grade bainite steel having an ultra-high yield ratio in each ofExamples 1-14 and the comparative steel in each of Comparative Examples1-10.

TABLE 2-1 Step (2) Initial rolling Final rolling Step (3) Step (4)Heating temperature of temperature of Coiling Cold rolling Steeltemperature finishing finishing Cooling rate temperature reduction rateNo. grade (° C.) rolling (° C.) rolling (° C.) (° C./s) (° C.) (%) Ex. 1A 1180 1140 920 70 500 50 Ex. 2 B 1205 1165 945 100 525 55 Ex. 3 C 12101170 950 50 570 80 Ex. 4 D 1190 1150 930 90 480 65 Ex. 5 E 1155 1115 900100 450 70 Ex. 6 F 1190 1150 930 110 510 60 Ex. 7 G 1240 1200 905 40 53575 Ex. 8 H 1230 1190 910 30 580 55 Ex. 9 I 1255 1215 915 150 470 60 Ex.10 J 1195 1155 935 120 515 65 Ex. 11 K 1205 1165 900 100 525 55 Ex. 12 L1175 1135 915 140 495 50 Ex. 13 M 1230 1190 905 130 550 60 Ex. 14 N 12401200 950 150 460 65 Comp. Ex. 1 O 1170 1130 910 50 490 50 Comp. Ex. 2 P1225 1185 915 100 545 70 Comp. Ex. 3 Q 1235 1195 925 120 495 65 Comp.Ex. 4 R 1215 1175 920 100 535 60 Comp. Ex. 5 S 1235 1195 900 80 505 55Comp. Ex. 6 T 1230 1190 910 90 550 50 Comp. Ex. 7 A 1180 1140 920 70 50070 Comp. Ex. 8 B 1205 1165 945 110 525 55 Comp. Ex. 9 F 1190 1150 930 90510 60 Comp. Ex. K 1205 1165 900 100 525 60 10

TABLE 2-2 Step (5) Step (e) Step (c) Cooling Cooling Step (d) time atStep (b) at Cooling controlled Step (a) soaking temperature temperaturecooling stage Heating Soaking First slow Second at fast Third Self- forself- rate at heating Soaking time at cooling cooling cooling coolingcooling temperature temperature stage temperature stage rate stage ratestage rate rise rise No. (° C./s) (° C.) (min) (° C./s) (° C.) (° C./s)Q (° C.) (° C./s) (° C.) (s) Ex. 1 5 850 2.0 5 740 55 97 365 2 108 10Ex. 2 15 860 3.0 8 730 50 103 382 2 119 34 Ex. 3 45 900 4.5 10 775 45 93410 1 65 40 Ex. 4 10 840 1.5 7 755 55 108 390 2.5 120 22 Ex. 5 20 8651.5 12 737 50 98 385 2 100 23 Ex. 6 5 855 3.5 11 741 50 100 382 2 112 22Ex. 7 20 875 2.5 4 762 60 89 397 1 58 30 Ex. 8 35 893 4.0 15 755 35 95405 1.5 65 28 Ex. 9 30 862 1.5 13 725 45 94 400 1.5 51 14 Ex. 10 10 8462.0 10 735 45 107 405 2.5 115 30 Ex. 11 15 868 1.0 8 746 50 91 380 1 8020 Ex. 12 40 887 2.5 7 750 40 100 400 2 98 25 Ex. 13 25 858 3.0 7 725 45104 379 2.5 98 12 Ex. 14 20 880 2.5 5 745 55 93 393 1.5 92 36 Comp. 30860 2.5 5 740 25 122 391 4.5 123 12 Ex. 1 Comp. 25 863 3.0 8 745 45 85386 1 64 20 Ex. 2 Comp. 10 858 2.0 7 735 40 101 396 2.5 111 18 Ex. 3Comp. 15 866 1.5 10 733 42 99 382 3.5 83 10 Ex. 4 Comp. 5 854 2.0 15 72050 84 378 71 30 Ex. 5 Comp. 20 857 2.5 10 725 30 107 393 3 111 25 Ex. 6Comp. 5 852 4.0 7 740 20 97 405 2 48 10 Ex. 7 Comp. 35 860 3.5 8 730 30103 450 2.5 45 10 Ex. 8 Comp. 40 855 3.0 5 741 35 100 382 1 112 10 Ex. 9Comp. 30 868 1.5 10 746 45 91 380 3 80 20 Ex. 10 Note: in the abovetable, Q = C × 180 + Si × 10 + Mn × 30 + Ni × 50 + Cr × 15 + Mo × 15 + B× 2000, whereineach element in the formula represents the value in frontof the percent sign in the mass percentage of the element.

Relevant mechanical performance tests were performed on the GPa-gradebainite steel having an ultra-high yield ratio in each of Examples 1-14and the comparative steel in each of Comparative Example 1-10. Themechanical performance test results of the Examples and ComparativeExamples are listed in Table 3. The relevant performance test methodsare described as follows.

The resulting GPa-grade bainite steel having an ultra-high yield ratioin each of Examples 1 14 and the comparative steel in each ofComparative Example 1-10 were sampled respectively. A transverse JIS 5#tensile sample was used to determine the yield strength and tensilestrength of the steel, and the middle area of the sheet was used todetermine the hole expansion rate and bending performance of the steel.

The hole expansion rate of the steel was determined in a hole expandingtest, wherein a test piece with a hole in the center was pressed into adie with a punch to expand the central hole of the test piece until theedge of the hole in the plate necked or through-plate cracks appeared.Since the manner for preparing the original hole in the center of thetest piece and the quality of the corresponding edge of the originalhole have a great influence on the test result of the hole expansionrate, the test and test method were implemented according to the testmethod of hole expansion rate specified in the ISO/DIS 16630 standard.The original hole in the center was in the form of a punched hole(corresponding to the processing method for an original hole having theworst edge quality). The 180° bending test was implemented using themethod for determining bending performance in the GB/T232-2010 standard(bending diameter d=1a).

Table 3 lists the test results of the mechanical performances of theGPa-grade bainite steel having an ultra-high yield ratio in each ofExamples 1-14 and the comparative steel in each of Comparative Examples1-10.

TABLE 3 Mechanical performances Yield strength Tensile strength YieldElongation Hole expansion rate 180º No. (MPa) (MPa) ratio (%) (%)bending Ex. 1 959 1003 0.96 9.3 68.2 d = 1a qualified Ex. 2 933 10220.91 9.7 62.2 d = 1a qualified Ex. 3 927 1010 0.92 9.8 62 d = 1aqualified Ex. 4 936 1038 0.90 9.6 56.7 d = 1a qualified Ex. 5 918 10190.90 10.5 58.4 d = 1a qualified Ex. 6 966 1031 0.94 9.5 65.4 d = 1aqualified Ex. 7 924 1016 0.91 9.6 61.3 d = 1a qualified Ex. 8 922 10260.90 10.5 61.8 d = 1a qualified Ex. 9 915 1015 0.90 11 55.5 d = 1aqualified Ex. 10 948 1020 0.93 9.7 61 d = 1a qualified Ex. 11 911 10080.90 10.7 55.9 d = 1a qualified Ex. 12 942 1037 0.91 10.3 59.3 d = 1aqualified Ex. 13 915 1008 0.91 10.8 57 d = 1a qualified Ex. 14 941 10050.94 9.4 66.2 d = 1a qualified Comp. 1075 1158 0.93 7.8 44.2 d = 1acracked Ex. 1 Comp. 849 942 0.90 11.2 68.8 d = 1a Ex. 2 qualified Comp.1008 1098 0.92 8.2 51.9 d = 1a cracked Ex. 3 Comp. 1031 1129 0.91 8 47.7d = 1a cracked Ex. 4 Comp. 875 977 0.90 10.8 66.9 d = 1a Ex. 5 qualifiedComp. 1020 1147 0.89 8.9 50.3 d = 1a cracked Ex. 6 Comp. 802 999 0.8011.9 43.9 d = 1a cracked Ex. 7 Comp. 781 996 0.78 12.4 40.1 d = 1acracked Ex. 8 Comp. 974 1035 0.94 8.8 52.2 d = 1a cracked Ex. 9 Comp.887 1002 0.89 11.6 53.4 d = 1a cracked Ex. 10

As it can be seen from Table 3, as compared with the comparative steelsin Comparative Examples 1-10, the mechanical performances of theGPa-grade bainite steels having an ultra-high yield ratio in Examples1-14 according to the present disclosure are obviously better. TheGPa-grade bainite steels having an ultra-high yield ratio in Examples1-14 according to the present disclosure have an ultra-high yield ratio,an ultra-high strength and excellent hole-expanding and bendingperformances at the same time, with a tensile strength of ≥980 MPa, ayield strength of ≥900 MPa, a yield ratio of ≥0.9, and a hole expansionrate of ≥55%.

In some individual preferred embodiments, as in Example 1, the GPa-gradebainite steel having an ultra-high yield ratio in Example 1 has a yieldstrength of ≥950 MPa, and a yield ratio of ≥0.95. That is, it has anultra-high yield ratio and an ultra-high yield strength.

FIG. 1 is a photograph at 3000× magnification showing the microstructureof the GPa-grade bainite steel of Example 1.

As shown by FIG. 1 , the microstructure of the matrix of the GPa-gradebainite steel in Example 1 is acicular lower bainite because it wascooled to the lower bainite phase region (the cooling temperature of thefast cooling met the requirement of the present disclosure) at asufficiently fast cooling rate (the second cooling rate met therequirement of the present disclosure) at the fast cooling stage. Inaddition, because the cooling rate at the controlled cooling stage forself-temperature rise was suitable (the third cooling rate met therequirement of the present disclosure), the structure also contains afine nano-, submicron- or micron-scale granular carbide precipitatephase that has been dispersively precipitated. The phase proportion ofthe acicular lower bainite is ≥90%; the total phase proportion of thegranular carbide precipitate phase+the acicular lower bainite is ≥99%;and the maximum diameter of the granular carbide precipitate is ≤2 μm.

FIG. 2 is a photograph at 3000× magnification showing the microstructureof the comparative steel in Comparative Example 7.

As shown by FIG. 2 , because the cooling rate was insufficient (thesecond cooling rate did not meet the requirement of the presentdisclosure) when the comparative steel in Comparative Example 7 wascooled at the fast cooling stage, bainite phase transformation occurredin the comparative steel at high temperature before it was cooled to thelower bainite phase region. Although it was still cooled to anappropriate temperature of the lower bainite phase region eventually,its microstructure is still dominated by massive equiaxed bainite,nearly free of acicular lower bainite, and the carbide precipitate isalso not fine and uniform enough.

FIG. 3 is a photograph at 1000× magnification showing the microstructureof the comparative steel in Comparative Example 8.

As shown by FIG. 3 , although the cooling rate was appropriate (thesecond cooling rate met the requirement of the present disclosure) whenthe comparative steel in Comparative Example 8 was cooled at the fastcooling stage, the cooling temperature of the fast cooling was too high(the cooling temperature at the fast cooling stage did not meet therequirement of the present disclosure). As a result, its microstructurealmost consists of massive equiaxed bainite, nearly free of acicularlower bainite, and the carbide precipitate is also not fine and uniformenough.

To sum up, as it can be seen, by designing the chemical compositionappropriately according to the present disclosure, a GPa-grade bainitesteel having an ultra-high yield ratio that has a tensile strength of≥980 MPa, a yield strength of ≥900 MPa, a yield ratio of ≥0.9, and ahole expansion rate of ≥55% can be obtained. The GPa-grade bainite steelhas an ultra-high yield ratio, an ultra-high strength, and excellenthole-expanding and bending performances at the same time. It can be usedto prepare automotive structural parts, and realize the new designconcept of “green-safety” for automobiles. It has good popularizationprospect and application value.

The annealing process according to the present disclosure plays a keyrole for the performances of the steel. The annealing process comprisesa heating stage, a soaking stage, a slow cooling stage, a fast coolingstage, a controlled cooling stage for self-temperature rise, and an aircooling stage. By designing the process appropriately and controllingrelevant process parameters, the GPa-grade bainite steel having anultra-high yield ratio can be obtained.

Correspondingly, the manufacturing method according to the presentdisclosure employs a unique production process. Particularly, the aboveannealing process is utilized to guarantee the performances of theresulting GPa-grade bainite steel. The resulting GPa-grade bainite steelnot only has ultra-high strength and yield ratio, but also has excellenthole-expanding and bending performances.

In addition, the ways in which the various technical features of thepresent disclosure are combined are not limited to the ways recited inthe claims of the present disclosure or the ways described in thespecific examples. All the technical features recited in the presentdisclosure may be combined or integrated freely in any manner, unlesscontradictions are resulted.

It should also be noted that the Examples set forth above are onlyspecific examples according to the present disclosure. Obviously, thepresent disclosure is not limited to the above Examples. Similarvariations or modifications made thereto can be directly derived oreasily contemplated from the present disclosure by those skilled in theart. They all fall in the protection scope of the present disclosure.

1. A GPa-grade bainite steel having an ultra-high yield ratio,comprising the following chemical elements in mass percentages inaddition to Fe and unavoidable impurities: C: 0.12-0.24%; Si: 0.2-0.5%;Mn: 1.3-2.0%; B: 0.001-0.004%; Al: 0.01-0.05%; at least one of Cr, Nb,Ti and Mo, wherein Cr≤0.4%, Nb≤0.06%, Ti≤0.1%, Mo≤0.4%.
 2. The GPa-gradebainite steel having an ultra-high yield ratio according to claim 1,wherein the mass percentages of the chemical element are: C: 0.12-0.24%;Si: 0.2-0.5%; Mn: 1.3-2.0%; B: 0.001-0.004%; Al: 0.01-0.05%; at leastone of Cr, Nb, Ti and Mo, wherein Cr≤0.4%, Nb≤0.06%, Ti≤0.1%, Mo≤0.4%; abalance of Fe and other unavoidable impurities.
 3. The GPa-grade bainitesteel having an ultra-high yield ratio according to claim 1, wherein themass percentages of the chemical elements satisfy at least one of: C:0.15-0.20%, Mn: 1.6-2.0%.
 4. The GPa-grade bainite steel having anultra-high yield ratio according to claim 1, wherein among the otherunavoidable impurities: P≤0.015%; and/or S≤0.004%.
 5. The GPa-gradebainite steel having an ultra-high yield ratio according to claim 1,further comprising at least one of the following chemical elements:0<Cu≤0.2%, 0<Ni≤0.2%, 0<V≤0.2%, 0<Ce≤0.2%.
 6. The GPa-grade bainitesteel having an ultra-high yield ratio according to claim 5, wherein itsatisfies 0.18≤M≤0.27, wherein M=Cr/2.5+Ti+V/5+Nb/1.7+Mo/1.7, whereinCr, V, Nb, Ti and Mo each represent a value in front of a percent signin the mass percentage of each chemical element; and/or 0.20≤C_(b)≤0.27,wherein an equivalent bainite carbon contentC_(b)=C−(Mo+Nb)/8−(Ti+V)/4−Cr/12+Ni/10+Mn/20+B×10, wherein each elementin the above formula represents a value in front of a percent sign inthe mass percentage of the element.
 7. The GPa-grade bainite steelhaving an ultra-high yield ratio according to claim 1, wherein itsmicrostructure is mainly acicular lower bainite, and a phase proportionof the acicular lower bainite is ≥90%.
 8. The GPa-grade bainite steelhaving an ultra-high yield ratio according to claim 7, wherein itsmicrostructure further comprises a nano-, submicron- or micron-scalegranular carbide precipitate phase that is precipitated dispersively,and a total phase proportion of the granular carbide precipitatephase+acicular lower bainite is ≥99%; preferably, the granular carbideprecipitate has a maximum diameter of ≤2 μm.
 9. The GPa-grade bainitesteel having an ultra-high yield ratio according to claim 1, wherein ithas a tensile strength of ≥980 MPa, a yield strength of ≥900 MPa, ayield ratio of ≥0.9, and a hole expansion rate of ≥55%; preferably ayield strength of ≥950 MPa, and a yield ratio of ≥0.95.
 10. An annealingprocess for the GPa-grade bainite steel having an ultra-high yield ratioaccording to claim 1, comprising steps of: (a) Heating a strip steel toa soaking temperature Ts at a heating rate of ≤50° C./s at a heatingstage, wherein Ts is 840-900° C.; (b) Holding the temperature Ts for 5minutes or less at a soaking stage; (c) Cooling to (Ts-80) to (Ts-140) °C. at a first cooling rate of ≤15° C./s at a slow cooling stage; (d)Cooling to (Ts-490) to (Ts-440) ° C. at a second cooling rate of≥(130-Q)° C./s at a fast cooling stage; (e) Cooling at a third coolingrate for 10-40 s at a controlled cooling stage for self-temperaturerise, wherein [(Q-80)/12]≤third cooling rate≤[(Q-80)/8]; (f) Finally,cooling the strip steel in air to room temperature at an air-coolingstage; wherein Q=C×180+Si×10+Mn×30+Ni×50+Cr×15+Mo×15+B×2000.
 11. Amanufacturing method for a GPa-grade bainite steel having an ultra-highyield ratio, comprising steps of: (1) Smelting and casting; (2) Hotrolling; (3) Post-rolling cooling and coiling; (4) Pickling and coldrolling. (5) The annealing process according to claim
 10. 12. Themanufacturing method according to claim 11, wherein in the step (2), aheating temperature is controlled at 1150-1260° C.; an initial rollingtemperature of finishing rolling is controlled at 1100-1220° C.; and afinal rolling temperature of finishing rolling is controlled at 900-950°C.
 13. The manufacturing method according to claim 11, wherein in step(3), a cooling rate is controlled at 30-150° C./s, and a coilingtemperature is controlled at 450-580° C.
 14. The manufacturing methodaccording to claim 11, wherein in step (4), a cold rolling reductionrate is controlled at ≥50%.
 15. The manufacturing method according toclaim 11, wherein the GPa-grade bainite steel having an ultra-high yieldratio comprising the following chemical elements in mass percentages inaddition to Fe and unavoidable impurities: C: 0.12-0.24%; Si: 0.2-0.5%;Mn: 1.3-2.0%; B: 0.001-0.004%; Al: 0.01-0.05%; at least one of Cr, Nb,Ti and Mo, wherein Cr≤0.4%, Nb≤0.06%, Ti≤0.1%, Mo≤0.4%.
 16. TheGPa-grade bainite steel having an ultra-high yield ratio according toclaim 2, wherein the mass percentages of the chemical elements satisfyat least one of C: 0.15-0.20%, and Mn: 1.6-2.0%; and/or among the otherunavoidable impurities: P≤0.015%; and/or S≤0.004%; and/or the GPa-gradebainite steel having an ultra-high yield ratio further comprises atleast one of the following chemical elements: 0<Cu≤0.2%, 0<Ni≤0.2%,0<V≤0.2%, 0<Ce≤0.2%.
 17. The GPa-grade bainite steel having anultra-high yield ratio according to claim 16, wherein it satisfies0.18≤M≤0.27, wherein M=Cr/2.5+Ti+V/5+Nb/1.7+Mo/1.7, wherein Cr, V, Nb,Ti and Mo each represent a value in front of a percent sign in the masspercentage of each chemical element; and/or 0.20≤C_(b)≤0.27, wherein anequivalent bainite carbon contentC_(b)=C−(Mo+Nb)/8−(Ti+V)/4−Cr/12+Ni/10+Mn/20+B×10, wherein each elementin the above formula represents a value in front of a percent sign inthe mass percentage of the element.
 18. The annealing process for theGPa-grade bainite steel having an ultra high yield ratio according toclaim 10, wherein the mass percentages of the chemical element of theGPa-grade bainite steel having an ultra-high yield ratio are: C:0.12-0.24%; Si: 0.2-0.5%; Mn: 1.3-2.0%; B: 0.001-0.004%; Al: 0.01-0.05%;at least one of Cr, Nb, Ti and Mo, wherein Cr≤0.4%, Nb≤0.06%, Ti≤0.1%,Mo≤0.4%; a balance of Fe and other unavoidable impurities.
 19. Theannealing process for the GPa-grade bainite steel having an ultra highyield ratio according to claim 18, wherein the mass percentages of thechemical elements satisfy at least one of C: 0.15-0.20%, and Mn:1.6-2.0%; and/or among the other unavoidable impurities: P≤0.015%;and/or S≤0.004%; and/or the GPa-grade bainite steel having an ultra-highyield ratio further comprises at least one of the following chemicalelements: 0<Cu≤0.2%, 0<Ni≤0.2%, 0<V≤0.2%, 0<Ce≤0.2%.
 20. Themanufacturing method according to claim 15, wherein the mass percentagesof the chemical element of the GPa-grade bainite steel having an ultrahigh yield ratio are: C: 0.12-0.24%; Si: 0.2-0.5%; Mn: 1.3-2.0%; B:0.001-0.004%; Al: 0.01-0.05%; at least one of Cr, Nb, Ti and Mo, whereinCr≤0.4%, Nb≤0.06%, Ti≤0.1%, Mo≤0.4%; a balance of Fe and otherunavoidable impurities.